1. Field of the Invention
The present invention relates to a negative electrode of a power storage device and a power storage device.
2. Description of the Related Art
In recent years, energy storage devices such as lithium-ion secondary batteries, lithium-ion capacitors, and air cells have been developed.
In addition, in a power storage device, a positive electrode and a negative electrode are each provided with a plurality of protrusions so as to increase the capacity; in such a power storage device, a top portion of the plurality of protrusions of the positive electrode and the negative electrode is provided with an insulator in order to reduce pressure applied to a separator between the positive electrode and the negative electrode when the volume of the positive electrode and the negative electrode increases owing to charge/discharge (see Patent Documents 1 to 3).
As an electrode of a lithium ion battery which is integrated on a silicon chip, submicron-diameter silicon pillars formed over an n-type silicon wafer is studied (see Patent Document 4). In Patent Document 4, a reactive ion etching is used to form the pillars. Further, Patent Document 4 discloses that photolithography may be used.
An electrode for a power storage device is manufactured by providing an active material over and in contact with a surface of a current collector. As a negative electrode active material, a material which can occlude and release ions functioning as carriers (hereinafter referred to as carrier ions), such as carbon or silicon, is used. For example, silicon or phosphorus-doped silicon can occlude about four times as many carrier ions as carbon and thus has higher theoretical capacity than carbon and is advantageous in increasing the capacity of a power storage device.
However, when the amount of carrier ions which are occluded is increased, the volume of an active material greatly changes in accordance with occlusion and release of carrier ions in charge/discharge cycle, resulting in lower adhesion between a current collector and silicon and deterioration in battery characteristics due to charge/discharge. Accordingly, a layer formed using silicon is formed over a current collector and a layer formed using a graphite is formed over the layer formed using silicon, thereby reducing deterioration in battery characteristics due to expansion and contraction of the layer formed using silicon (see Patent Document 5).
Silicon has lower electric conductivity than carbon; thus, by covering surfaces of silicon particles with a graphite and forming an active material layer including the silicon particles over a current collector, a negative electrode in which the resistivity of the active material layer is reduced is manufactured.
In recent years, the use of a graphene as a conductive electronic material in semiconductor devices has been studied. A graphene refers to a sheet of one atomic layer of carbon molecules having sp2 bonds.
A graphene is chemically stable and has favorable electric characteristics and thus has been expected to be applied to channel regions of transistors, vias, wirings, and the like included in the semiconductor devices. In addition, particles of an active material are coated with a graphite or a graphene in order to increase the conductivity of a material for an electrode in a lithium-ion battery (see Patent Document 6).